1. Field of Invention
The present invention relates to electro-blowing or blowing-assisted electro-spinning technology, and more particularly to a spinneret format and to a process for post-treatment of membranes formed from such technology, including a cleaning method and apparatus for electro-blowing or blowing-assisted electro-spinning technology.
2. Discussion of the Background
One technique conventionally used to prepare fine polymer fibers is the method of electro-spinning. When an external electrostatic field is applied to a conducting fluid (e.g., a charged semi-dilute polymer solution or a charged polymer melt), a suspended conical droplet is formed, whereby the surface tension of the droplet is in equilibrium with the electric field. Electro-spinning occurs when the electrostatic field is strong enough to overcome the surface tension of the liquid. The liquid droplet then becomes unstable and a tiny jet is ejected from the surface of the spinneret tip. As it reaches a grounded target, the jet stream can be collected as an interconnected web of fine sub-micron size fibers. The resulting films from these non-woven nanoscale fibers (nanofibers) have very large surface area to volume ratios.
The electro-spinning technique was first developed by Zeleny[1] and patented by Formhals[2], among others. Much research has been done on how the jet is formed as a function of electrostatic field strength, fluid viscosity, and molecular weight of polymers in solution. In particular, the work of Taylor and others on electrically driven jets has laid the groundwork for electro-spinning[3]. Although potential applications of this technology have been widely mentioned, which include biological membranes (substrates for immobilized enzymes and catalyst systems), wound dressing materials, artificial blood vessels, aerosol filters, and clothing membranes for protection against environmental elements and battlefield threats[4-26]. The major technical barriers for manufacturing nanofibers by electro-spinning are the low speed of fabrication and the limitation of process to polymer solutions, which can be summarized as follows:
1. The first barrier involves electrical field interferences between adjacent electrodes (or spinning jets), which limit the minimum separation distance between the electrodes or the maximum density of spinnerets that can be constructed in the multiple jet electro-spinning die block. Recently, scientists at STAR (Stonybrook Technology and Applied Research) and at Stony Brook University developed a unique esJets™ technology and the new technology can overcome this hurdle (B. Chu, B. S. Hsiao and D. Fang, Apparatus and methods for electro-spinning polymeric fibers and membranes. U.S. Pat. No. 6,713,011 (2004)).
2. The second barrier is related to the low throughput of the individual spinneret. In other words, as the fiber size becomes very small, the yield of the electro-spinning process becomes very low.
3. The third barrier is limited by the capability for continuous operation over extended periods of time and automatic cleaning of multiple spinnerets with minimal labor involvement.
4. The last barrier of electro-spinning is due to the limitation of solution processing, where the use of solvent severely hinders the industrial applicability of the technique. The current invention is aimed to overcome (2)-(4) technical hurdles of the conventional electro-spinning technology, as well as to affect (1) the flow of fluid jet streams by gas-blowing.
U.S. patent application Ser. No. 10/674,464 (2003) (B. Chu, B. S. Hsiao, D. Fang, A. Okamato, Electro-Blowing Technology for Fabrication of Fibrous Articles and Its Applications of Hyaluronan) was filed by STAR based on the concept of blowing-assisted electro-spinning from polymer solutions and the preparation of hyaluronic acid (HA) nanofibers using this technology. The entire content of U.S. Ser. No. 10/674,464 is hereby incorporated by reference.
PCT application WO 03/080905 (2003), filed by scientists at NanoTechniques, proposes a high-throughput production method based partially on electro-spinning: A manufacturing device and the method of preparing for the nanofibers by electro-blown spinning process. However, there are several drawbacks in this disclosed technology.
1. It only deals with the processing of polymer solutions.
2. It does not fully utilize the electrical field to achieve a sufficiently large spin-draw ratio during blowing, thus, they cannot produce smaller size diameter fibers (e.g., fibers of less than 300 nm in diameter).
3. It cannot sustain a long-term operation capability (e.g., >5 days) because the unavoidable polymer deposits (accumulations) on the spinneret will pose a major problem for sustained operation. No scheme was proposed to resolve this difficulty.
3. General Consideration
Electro-spinning and melt-blowing are established technologies. In electro-spinning, the applied electric field is the main driving force responsible for the production of sub-micron diameter fibers; while in melt-blowing, the mechanical gas-flow shear/elongational and drag force is the main driving force responsible for the production of micron diameter fibers. The advantage of the electro-spinning process is the production capability of smaller sub-micron diameter fibers with sizes in the 10 nm-micron diameter size range, but the disadvantage has been the relatively lower production throughput. The advantage of the melt-blowing process is the relatively high-production throughput, while the disadvantage is the production of relatively larger fiber diameters in the micron diameter size range.
The combination of an applied electric field and a flowing gas stream is a natural extension of such technologies. However, the successful implementation of a combination of the two technologies is in making a distinction between spinning a polymer in the molten state (e.g., melt-blowing) or in the solution state (e.g., electro-spinning). In melt-blowing, the resistance to spin-draw of the polymer-melt jet stream is closely related to the anisotropic crystallization and solidification processes as well as the speed of the gas (air, in most cases) that provides the mechanical shear and drag force, whereas in electro-spinning of a polymer solution, the resistance to spin-draw a solution jet is closely related to the solvent evaporation rate, in addition to polymer solidification and possible crystallization.
It is clear that the jet instability due to electrical repulsion inside the jet stream is an essential means to produce the very large spin-draw ratio (in the absence of bifurcation), necessary for the production of truly sub-micron diameter fibers. Then, the essence of a temperature-controlled gas-blowing assisted electro-spinning process is to use the gas, not only as a shear/elongational and drag force, but also to control the polymer solidification/crystallization from polymer melts as well as the rate of solvent evaporation, together with solidification from polymer solutions. In both processes that use a combination of electrical force and gas-blowing force, as well as in the established electro-spinning and melt-blowing technologies, sustained operations over long time periods have been a major drawback in practice. For example, even with the established melt-blowing technology, provisions are made to replace entire banks of a melt-blowing unit in order to be able to maintain continuous operation. For solution spinning, the solidified polymer around the spinneret is often below the polymer glass transition temperature. Such accumulations around the spinneret head cannot be routinely removed by blowing gas. Thus, solution spinning can impose a more serious problem. For the gas-blowing dominated spinning process, the spinneret diameter may have to be relatively smaller because of more limited spin-draw ratio.
It should be noted that spinning is a physical process. In electro-spinning, the spin-draw ratio is of the order of one million. Consequently, for a production rate of ˜6 g of polymer/20 hrs/spinneret by using a 10 wt % polymer solution (assuming a density of 1 g/cm3) and an effective cross-sectional area of 0.04 mm2 for the spinneret hole, the initial fluid velocity is ˜75 m/hr. With a spin-draw ratio of one million, a final fiber cross-sectional area of 0.04 μm2 (corresponding to a fiber diameter of about 200 nm) and remembering that the polymer solution contains 90% solvent that will be evaporated, the final fiber speed reaching the collector is about 750 km/hr, about the speed of an airplane. Thus, if one considers increasing the production rate per jet by a factor of only 10, the fiber speed will break the sound barrier, long before the fiber cross-section can be reduced to much smaller than the cross-sectional area of 0.04 μm2. This illustration simply implies that, for a single jet stream from each spinneret, i.e., without bifurcating the jet stream into multiple jet streams, the generation of very small fiber diameters cannot be accomplished only by using the mechanical gas shearing/elongational and drag force (as in melt-blowing). It has to be achieved with the additional electrical force. Furthermore, a gas-flow rate beyond the sound barrier is impractical, not to mention the high-energy consumption needed to produce a gaseous stream at very high velocities. Thus, there is a need for practical solutions to the above, by increasing the number of spinnerets with robust operations, and for smaller diameter fibers, the process is a gas-flow assisted electro-spinning process. It should also be noted that more effective operations require high polymer solution concentrations. Thus, a polymer melt, having no solvent to evaporate, is an effective way to increase the production rate, if the polymer melt viscosity can be reduced to the proper range. The limitation for melt-spinning using a combination of electrical and mechanical (gas-blowing) forces is related to high temperature operations and the nature of temperature control.
Methods for the post-treatment of electrospun (or electroblown) membranes are needed to provide new structures (crystallinity and crystal form), new morphologies (multiple distributions of porosity, preferred fiber orientation), and improved membrane properties (mechanically and thermally stable in dry and wet environments, electrical conductivity). The capability to manipulate the structure and morphology of electro-spun membranes using such post-treatments can provide means to control and enhance the physical properties for varying applications, such as improved thermal and mechanical stability and electrical conductivity for fuel cell and battery applications, controlled porosity distributions for cell attachment and proliferation in tissue engineering, and new separation capability for many applications such as filtration.